1. Field of the Invention
The present invention relates to an air-pressure impact type drilling method and an apparatus for the same in which a piston of an air-actuated hammer drill is moved up and down by pressure air to produce impact energy which can be used to give repetitive impact vibrations to a drill bit, and, in particular, to such air-pressure impact type drilling method and apparatus for the same which can be used to drill through conglomerates, rocks and the like under muddy water.
2. Description of the Prior Art
An air-actuated down-the-hole hammer drill comprises a tubular casing for supporting a drill bit slidably in the axial direction at the tip end thereof, and a piston which is supported slidably in the axial direction thereof within the tubular casing to hit against the drill bit at the lower end thereof. There are formed two pressure chambers upwardly and downwardly of the piston respectively and these two up and down pressure chambers are alternately pressurized by pressure air from an external source to thereby move up and down the piston successively.
Conventionally, the pressure air used to operate a piston is jetted out from the tip end of a bit and the jetted air cleans the edge of the bit and at the same time blows cut-out slimes and the like up onto the ground while drilling (which is referred to as a dry boring method). The air-actuated hammer drill can provide high impact forces due to use of compressed air and is normally used to drill through the rocks.
However, when it is used under water, the conventional air pressure impact hammer drill of this type is found disadvantageous in several points. For example, in the case of drilling using muddy water or drilling under sea, a river, a dam or the like (which is referred to as a wet boring method), water head pressure is applied against the tip end of the bit to cause the muddy water or the like to flow backward into the air pressure chambers, thereby making the impact hammer drill inoperable. Also, even when the water head pressure does not reach the jetted-air pressure, the operating air pressure is decreased by the water head pressure. As result of this, the impact energy of the piston is lowered accordingly and thus the piston is not able to hit against the bit properly so that the drilling efficiency of the air hammer drill is worsened.